Many people have developed large immersive displays based on rear projection techniques. These systems generally incorporate a projection screen or multiple projection screens in large CRT-based projectors which project images from a computer onto the screen. The user or users generally wear flicker stereo glasses and see images on the projection screens. The images on the projection screens are refreshed at 120 hertz (or some other higher than normal vertical rate) and the flicker glasses provide the user with different images to each eye by changing the opacity of the lens in front of each eye in sync with the accelerated vertical refresh of the monitor.
These systems vary in their configuration and may have between 1 and 6 projection screens in a single system. In multi-screen configurations the viewer may stand either in a corner at the junction between 2 screens or in a room composed of several projection screens with images projected onto the walls, floor and possibly even the ceiling. Each screen has an image projected on it by a projector which is oriented behind the screen—possibly with an intervening mirror—which relays the image and shortens the image path or folds in a convenient way. In single screen systems, the screen is either oriented at an angle relative to the viewer, much like a drafting table, or it may be oriented in a horizontal orientation so that it more closely resembles a table surface.
Some researchers, such as NRL have been doing similar work with non head-tracked interfaces. In these settings, the viewer or viewers all see a stereo pair and for the best view should stand in a small cluster near the place where the perspective is calculated from in the software.
These systems have been demonstrated on a number of occasions. One of the early public demonstrations was presented by Michael Deering of Sun Microsystems at Siggraph 1992. The Virtual Portal system incorporated three walls which were projection screens, each of which was approximately 10′.times.10′ and surrounded the user who wore stereo glasses and a head-tracker. The head-tracker is used so that the computer can update the images appropriately as the user moves around. For example, if there is a cube in front of the user on the projection screen, as the user moves around the images are updated in such way that the user may look at different sides of the cube by moving their head or walking around it.
In addition to large systems such as this, Michael Deering has also developed systems which are based on a flat, small monitor display as would typically accompany a workstation. Again, head-tracking is used so that the stereo images presented on the display give an illusion of an object floating in front of the user. The user can generally interact with these objects by use of a hand-tracker of some kind of a tracked wand. The tracking technology for these systems is generally either electromagnetic in nature or acoustic. Michael Deering, in particular, has generally used acoustic tracking for various reasons.
Other researchers have used magnetic tracking. An example of this would be the systems that have been developed by the University of Illinois at Chicago (Electronic Visualization Lab) which were demonstrated at Siggraph 1993. These systems use an electromagnetic tracker to track the location of a person in a room composed of a number of large screens. The advantage of electromagnetic tracking is that there are no “line-of-sight” requirements for the tracking technology to function correctly.
These systems generally have one person who wears both the stereo glasses which are standard flicker glasses for stereo at 120 hertz (approx. per eye) and the computer updates the perspective views for left and right eyes according to the location of this individual person. Thus, other people who are also wearing the stereo glasses will see a stereo image but it is offset because the images are being calculated from the head-tracked point of view rather than from their own point of view.
Consider the cube example: if there is a virtual cube and person A is standing on one side of the cube with the head-tracker and another person B is actually standing on the other side of the cube, both people in fact see the same image because only one view is presented and it is from the point of view of person A who has the head-tracker. The person B without the head-tracker sees an incorrect view. For person B to get a correct view, the tracker would have to be passed over to person B and the image calculated from B's point of view. Of course, then A is getting the wrong view which is not ideal. In general, when using these systems there has been the restriction that users who are passively viewing the system must stand close to the person with the head-tracker in order to get an incorrect but sort of acceptable perspective.
In a multi-screen projection environment, this limits the number of people who get a reasonable view to just a couple of people and also restricts the mobility of those who all must stay as a small group inside the room. This means that things like collaborative computing and collaborative design are difficult because one person is essentially in control of what everybody sees. In other systems, this is also an issue. For example, the responsive workbench, originally developed at GMD in Germany, is a flat table-top display and again head-tracking with stereo glasses is used so that one may see a stereo image on the table. If multiple people are looking at the table together, only those people who are very close to the person with the head-tracker see a reasonable view and as this person moves around, the passive viewer's perspectives are very distorted. This is particularly objectionable in situations such as training and situation awareness or planning. In a training exercise, a surgeon may be attempting to illustrate a procedure to a student and the student would like to be able to see the procedure from the correct perspective from their point of view. The surgeon also needs to be able to see the operation proceeding correctly from her own point of view and it is unacceptable for the view of either person to be incorrect.
Thus, in all present systems, a single viewer has both the head-tracker and the stereo glasses and this person essentially controls the view that all other participants will see. We have developed a system which eliminates this restriction and simultaneously provides the correct perspective to more than one person. This is a tremendous step forward for presenting a virtual environment in a large projection-type display to more than one person at a time which is very important for collaborative computing and collaborative design.